Cache build directory in GitHub Actions (#107)

.github/workflows/ci.yml

@@ -49,6 +49,11 @@ jobs:
       with:
         path: ~/.cargo/registry
         key: ${{ runner.os }}-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+    - name: Cache cargo build
+      uses: actions/cache@v3
+      with:
+        path: target
+        key: ${{ runner.os }}-cargo-build-${{ hashFiles('**/Cargo.lock') }}
     - name: Check formatting
       uses: actions-rs/cargo@v1
       with: